Today, resin porous membranes (hereinafter, also referred to simply as “porous membranes”) are applied widely to filters. Particularly, porous membranes made of polytetrafluoroethylene (PTFE), that is, PTFE porous membranes, have excellent heat resistance, chemical stability, electrical insulation, water repellency, and oil repellency, which are characteristics derived from PTFE. When the PTFE porous membranes are used as a filtering medium of a filter, dust hardly is generated, and both a low pressure loss and a high collection efficiency can be achieved at the same time. Thus, the PTFE porous membranes are used widely for filters, such as filters for clean rooms, filters for dust catchers, and water proof gas permeable filters and sound pressure adjusting filters used in electronic devices such as cellular phones. The PTFE porous membranes increasingly have been used as the water proof gas permeable filters in particular that can transfer sound while preventing liquid from contacting a transducer (a transmitter and a receiver) of a cellular phone.
When the PTFE porous membrane is used for any of these applications, particularly for an electronic device, the porous membrane is bonded directly to a base component constituting the device, such as a housing, in many cases. When the porous membrane is bonded to the base component, it is important to ensure the gas permeability of the porous membrane.
As common conventional methods for bonding the porous membrane to the base component, there can be mentioned a method in which an adhesive material processed into a predetermined shape is disposed on a surface of the porous membrane or a surface of the base component, and a method in which the porous membrane and the base component are welded thermally to each other, although the latter is possible only when the base component is made of thermoplastic resin. For example, JP 2003-503991 T (Document 1) discloses methods for bonding a PTFE porous membrane that will be used as a water proof gas permeable filter of an electronic device, such as a microphone and a buzzer. One is a method in which a cut adhesive tape is bonded to a porous membrane, and another is a method in which a thermoplastic adhesive, a thermosetting adhesive, or a reactive curable adhesive is applied directly to the porous membrane by a technique such as screen printing, gravure printing, spray coating, and powder coating (see 0030 etc. of Document 1).
In recent years, as electronic devices have been downsized and highly integrated, the porous membranes to be bonded to their base components are required strongly to be smaller. With an intent to achieve further downsizing and higher integration of the electronic devices, it also has been required to dispose electronic parts, such as a transducer, directly on a circuit board as well as to arrange the porous membrane in such a manner that the porous membrane covers the electronic parts. That is, it also has been required to bond the porous membrane directly to the circuit board.
However, it is difficult to bond the smaller porous membrane to the base component while ensuring the gas permeability of the porous membrane by using the conventional bonding methods because they have limitations in reducing the size of the adhesive material to be disposed on the porous membrane, and in reducing the amount of the adhesive applied to the porous membrane. Moreover, the size reduction of the porous membrane, on which the adhesive material is disposed (on which the adhesive is applied), makes it difficult to handle, and lowers the precision in bonding the porous membrane to the base component. Furthermore, when the adherend is a circuit board, it is not possible to bond the porous membrane thereto by thermal welding because circuit boards usually are made of thermosetting resin.